These are three gastropods from the family Vitrinidae on a human hand. Each of them belong to a different genus. All of them shows a different level of a reduction of its shell.
The upper one is Vitrina pellucida. This gastropod can completely withdraw into its shell and therefore it is called a snail.
Two others can not fully withdraw into their shells. They are semi-slugs.
The lower left is Eucobresia diaphana. Mantle covers apical part of the shell.
The lower right is Semilimax semilimax. Quite big mantle lobe is covering an apex. The reduction of the shell of Semilimax semilimax is the highest among these three species.
Genera in the family Vitrinidae shows the complete example of the shell reduction from the snails through semi-slugs to slugs.
Examples of snails: Phenacolimax, Vitrina.
Examples of semi-slugs: Eucobresia, Semilimax, Vitrinobrachium.
Example of a slug in Vitrinidae is Plutonia atlantica. It is a slug with an internal shell living on Azores.
References:
Species summary for Plutonia atlantica. AnimalBase, accessed 2 January 2014.
Species summary for Vitrina pellucida. AnimalBase, accessed 2 January 2014.
gastropods 